Getting an error for: * * * * * ?

[maalsol]

Calling the following the code using the expression * * * * * ?
var e = later.parse.cron(b, !0);
return later.schedule(e).isValid()

Causes the following error:

angular.min.js:formatted:2997 TypeError: Cannot read property 'constraint' of undefined
    at Object.e.compile (later.min.js:formatted:561)
    at Object.e.schedule (later.min.js:formatted:712)

Replacing any of the star symbols for a valid value returns correctly, however I believe that the expression I'm using is valid.

Any Ideas?